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Logic and Computation Seminar

Monday, April 14, 2008 - 4:30pm

Nate Ackerman

University of Pennsylvania

Location

University of Pennsylvania

DRL 4C8

One of the most significant partial results regarding Vaught's conjecture was made by Michael Morley in the 1970 paper "The Number of Countable Models". In this paper he proved that every sentence of L_{\omega_1,\omega} has either countably many, continuum many or \omega_1 many countable models. In the proof he showed that the countable models of such a sentence could be arranged in a natural way in what is now called a Vaught tree. He further showed that the number of models at any level of this tree is either countable or has size of the continuum. Hence, the only way that that Vaught's conjecture could fail is if there is a sentence of L_{\omega_1,\omega} whose Vaught tree has only countably many models at each level and has height \omega_1. In talk we will continue the presentation of a method which (with a few assumptions) allows us to construct for each ordinal \alpha a sentence of L_{\omega_1,\omega} whose Vaught tree has height approximately \alpha and which has only countably many models at each level. As such these sentences can be viewed as approximations to a counterexample to Vaught's conjecture. At the beginning of the talk we will review what was covered in the previous talks before continuing our discussion of a single tree. We will discuss how our multiple trees can be combined to give our desired results. This work is based on the main result of my PhD thesis.